The many attributes of gold as a functional material have long been recognized. Superior conductivity, relative corrosion resistance, ease of handling, and general elemental stability are but a few beneficial characteristics of this versatile element. However, gold's recent price per ounce is so expensive that most manufacturers of consumer products other than fine specialty electronics (which may necessitate pure gold contacts and other gold circuitry) must settle for gold plating as a means to keep the product's eventual unit price within acceptable market boundaries.
The electroplating industry has observed the realization that gold-plating is often an acceptable substitute for the use of pure gold in certain situations. Relatively pure gold (up to 99.9999% pure) is readily available and can easily be fashioned into highly soluble gold salts such as, for example potassium gold cyanide (PGC). These convenient salts can then be used to prepare plating baths by putting the PGC into solution under specified conditions. Gold plating traditionally gives a strong, hard deposit onto the plated substrate of choice which can vary widely. In this way the superior elemental properties of gold can be imparted upon base metals, alloys, other electroplate layers, or previously plated or otherwise treated non-metallic substrates.
However, certain physical properties of gold, such as its relative porosity, translate into problems when gold is plated onto a substrate. For instance, gold's porosity can create interstices on the plated surface. These very small spaces can contribute to corrosion or actually accelerate corrosion through the galvanic coupling of the gold layer with the underlying base metal plate layer or layers. This is apparently due to the fact that the base metal substrate and any accompanying underlying metal layers may be exposed to corrosive elements via the pores in the plated gold outer surface.
In the field of fashion eyewear, due to the significant realized cost savings, gold plate has often been used to plate the metal eyewear frames. Further, in the field of high-performance eyewear, such gold plating often becomes functional in nature. For example, if the eyewear is expected to be used by military personnel, the gold plating must pass certain rigorous corrosion tests, since the eyewear can be expected to be used in a variety of different climates and be exposed to various corrosive conditions (e.g. salt water, sea spray, fog, excessive humidity, perspiration, airborne pollutants, etc.). Similarly, various professional sporting activities can expose gold-plated eyewear to the same abovelisted corrosive conditions.
Conventional gold electroplating has not been found to be entirely satisfactory. For example, it has been observed in the industry that even the most-expensive gold-plating processes, over time, begin to exhibit signs of corrosion. It is believed that the base metal substrate, or the primary or secondary metal plate layers underlying the final gold-plate layer begin to oxidize if exposed to corrosive conditions for sufficient time periods. It is further believed that gold's natural properties, which include a certain inevitable degree of porosity, at least partially contribute to this corrosion problem. Therefore, there has been an ongoing effort in the electroplating industry to overcome the corrosion problem resulting from gold's natural porosity.
At least three different approaches to overcoming the corrosion problems have been attempted: 1) reducing the porosity of the coating, 2) inhibiting the galvanic effects caused by the electropotential differences of different metals, and 3) sealing the pores in the electroplated layer. Reducing the porosity has been studied extensively in the past. Pulse plating of the gold and utilization of various wetting/grain refining agents in the gold plating bath affect the gold structure and are two factors contributing to a reduction in gold porosity. Often regular carbon bath treatments and good filtration practices in the series of electroplating baths or tanks, combined with a preventive maintenance program help to maintain good metal deposition levels and correspondingly low levels of surface porosity. A certain degree of porosity, however, continues to remain.
Pore closure, sealing, and other corrosion inhibition methods have been tried in the field with limited success. Possible mechanisms using organic precipitates having corrosion inhibitive effects are known in the art. Many of these compounds were typically soluble in organic solvents and were deemed not to provide long term corrosion protection. Other methods of pore sealing or pore blocking are based on the formation of insoluble compounds inside the pores. Such insoluble complexes and precipitates as will be apparent to those skilled in the field are also potential candidates for pore blocking. However, attempts at forming stable nickel complexes have been unsuccessful.
Passivation, as the term is used in connection with the present invention, is defined as the process of pore and surface layer sealing. Passivation is commonly practiced in the electroforming industry on a large scale. Mandrels are typically made from stainless steel or nickel. Stainless steel readily forms a passive surface layer due to the chromium or nickel alloy additions. Both nickel and chromium form stable passive films which most likely consist of adsorbed or chemically combined oxygen (oxides). These films are thin, typically measured in nanometers, and are relatively uniform. The advantage of these thin films in electroplating is that they are somewhat conductive, allowing for further electro-deposition. However, the passive surface layer at the interface allows for easy separation of the electroform from the mandrel after the deposition is complete.
The stainless steel that is often used in critical corrosion resistant applications is usually passivated in a nitric acid solution (20%) with or without the addition of a dichromate ion, (Cr.sub.2 O.sub.7).sup.-2. Sodium or potassium salts of the dichromate are conventionally used in this operation with a thin film resulting which effectively provides corrosion resistance. The use of a plated dichromate ion layer is therefore not new in the electrochemical field. However, dichromates have traditionally been used only to mask, or create an intermediate pre-plate "masking" film layer on a base metal substrate when selective plating techniques are used.
For example, in the electronics industry, gold-plated circuitry is common. However, due to the high cost of gold, printed circuit boards and other parts are only selectively gold-plated. This intermediate dichromate process takes place before the final precious metal surface layer is plated. Dichromates are first applied to the base metal substrate, usually copper. The resulting dichromate layer left on the base metal substrate inhibits further plating by most metals, including gold. To then plate the desired final plate (e.g. gold) on the printed circuit, the dicromate layer must first be etched or otherwise selectively removed from the base metal. In these applications, the dichromate intermediate layer effectively acts as a plating inhibitor for the rest of the circuit board as the gold is then plated only in the etched pattern of the printed circuit. For examples, see U.S. Pat. Nos. 4,082,620 (Shurkiss) and 4,077,852 (Koontz, et al.).
Further, Sharfrin, et al. in Applied Surface Science, Vol. 4, (April, 1980) pp. 456-465, describe passivation of gold plating over copper base metal circuit boards in nuclear submarine navigational computers as a field repair measure.
Although the exact chemical mechanism remains unclear, it is believed that a corrosion problem from a galvanic mechanism involving the gold layer exists. For example, it is now believed that when a base metal substrate is plated with a primary nickel-containing electroplate, optionally followed with a secondary nickel-containing layer which is then plated with a final precious metal outer layer, galvanic coupling of the gold electroplate with the primary nickel or secondary nickel-containing layers occurs, resulting in an accelerated corrosion rate. The addition of a Pd/Ni layer over the primary nickel layer and under the final precious metal plate was tried in an attempt to decrease the electropotential difference betwen the primary nickel and the final outer precious metal plate layer. It was believed that the decreased electropotential would result in a reduced rate of corrosion. In fact, significant corrosion inhibition was observed. It is further believed that the Pd/Ni secondary plate layer therefore interrupts the galvanic couple between the gold and primary nickel layers. By the improved novel process of the present invention it was further discovered that the addition of the dichromate as a final post-treatment after the precious metal plate provided still greater corrosion resistance.
Therefore, in simple terms, passivation may be further defined as the formation of a film ( e.g. oxide) on a metal (anodic) surface, which will resist dissolution in a particular electrolyte thereby protecting the coated metal layers below. Such films may be penetrated by either electrolytic changes (e.g. chemical agents) to dissolve the film or by high electric potentials to induce transport of ions through the film to resume the anode dissolution reaction (corrosion).